In treatment for asthma and other respiratory problems, a dosing device such as an inhaler can be used to produce an aerosol mist or cloud of fine particles for inhalation into a patient's lungs. Typically inhalers are either manually-operated or breath-operated. Breath-operated inhalers have certain advantages in that the dose given is necessarily in synchronism with intake of breath by the patient, whereas manually operated devices require a patient to breathe in as a button is pressed, sometimes resulting in ineffective dosing due to the intake of breath being wrongly timed.
Breath-operated inhalers are known, for example U.S. Pat. No. 3,565,070 and WO98/52634. These inhalers are known as metered-dose inhalers (MDI's) and consist of a small canister containing medication with a metering valve and a valve stem. The MDI delivers a metered dose to the patient when the valve stem is depressed. However a large force is required to depress the valve stem and so release the dose, and a problem in the design of breath-operated MDI's is how to achieve release of such a large force using only a very small force available from the patient's breath. This problem is partly overcome by using a large spring to a sufficient force to actuate the device. The spring is compressed by the patient, either by a positive “cocking” process or automatically when the patient opens the mouthpiece cover. The energy stored in the spring is then released by a trigger operated by the patient's breath.
Components forming the trigger are limited in size by the dimensions of the MDI, which is a hand-held device. The trigger also needs to be able to open the valve reliably only when a dose is required and for the life of the device.
MDI's are beginning to use hydrofluroroalkanes (HFA's) as aerosol propellants within the canister. The HFA's need to be held at a pressure of around 5–6 bar, and certain valves can require forces of up to 50N for operation. This further increases the difficulties in achieving a trigger which can operate consistently and reliably.
The present invention aims to provide an inhaler with a dose actuation mechanism which consistently results in delivery of a dose from an inhaler in response to a patient's breath.